A radiation system of the type here under consideration is Application DE-OS 34 15 610, and its counterpart U.S. Pat. No. 4,620,146 particular reference being made to FIG. 5. This publication deals With an arrangement for determining moisture in a track-like object which is guided through a radiation system composed of horn-shaped transmitting and receiving antennas. The absorption of microwave radiation is in this case a measurement of the moisture content of the track.
Such an arrangement can also be used if, for example, a conveyor transporting bulk material is employed, instead of the track.
In connection with such measurements, microwave radiation components naturally appear which are scattered and/or reflected by objects in the vicinity and which reach the receiver horn antenna and lead to a distortion of the measured results. In the known device, the housing for the arrangement or the surfaces responsible for the undesired reflections and leakage radiation are therefore covered with an absorption material which is intended to provide an appropriate damping of the microwave leakage radiation.
In connection with the technical problems of absorption of vibrations, it is basically known from an article by H.-G. Haddenhorst, entitled "Durchgang von elektromagnetischen Wellen durch inhomogene Schichten (Teil II: Absorption von elektromagnetischen Wellen)"[Penetration of Non-homogenous Layers by Electromagnetic Waves (Part II: Absorption of Electromagnetic Waves)]in Zeitschrift for angewandte Physik [Journal of Applied Physics], Vol. VIII, Issue 6-1956, pp. 264 to 267, that there are various possibilities which basically depend on various physical effects.
Use is made of a suitable "absorption material" which, for example, achieves a damping effect, although over a relatively broad band, by transforming the arriving wave energy into heat or similar mechanisms, but this is not particularly effective. This may be the reason why, in the known embodiment in accordance with FIG. 5 of U.S. Pat. No. 4,620,146 an outward inclination is shown in addition to the absorption layer 18. Obviously it was realized in this case that the absorption layer 18 does not suffice to eliminate interference radiation in a satisfactory way, so that an "outward reflection" of the undesirable radiation component is achieved by means of the inclined outer surfaces, as shown by the dashed arrows.
A basically different possibility of the absorption of radiation consists in providing a resonator or resonating circuit based on the specific character of the radiation. However, the physical principle of resonance absorption results in a narrow band which can only be usefully employed if the damping of radiation of a correspondingly narrow bandwidth is involved. Otherwise it is possible to try to "widen" the frequency characteristics by the suitable coupling of a plurality of resonance absorbers with adjoining resonance frequencies. However, this unavoidably results in increased manufacturing costs.